The track structure of α-particles from the point of view of ionization-cluster formation in "nanometric" volumes of nitrogen

Abstract

Probability distributions of the size of ion clusters created in "nanometric" volumes of nitrogen by single alpha-particles of a gold-plated 241Am source, were measured and compared with those calculated by Monte Carlo methods in the same geometry. The diameter of the sensitive volumes had a mass per area of between 0.015 microgram/cm2 and 1.3 micrograms/cm2 which, for a material at unit density, corresponds to a nanometric target volume 0.15-13 nm in diameter. These nanometre sizes were simulated experimentally in a device called the Jet Counter. This consists of a pulse-operated valve which injects into an interaction chamber an expansion jet of molecular nitrogen gas, which is crossed by a narrow beam of alpha-particles. The resulting ions are counted and analyzed from the point of view of ionization cluster formation. The measured or calculated cluster size probabilities prove that the formation of ionization clusters along a "nanometre" track is governed by Poisson's law only in the case of very small target volumes, due to the contributions by secondary electrons. The present ionization cluster probabilities produced in "nanometric" volumes 0.15-13 nm in diameter, are the first ever determined experimentally and confirmed by Monte Carlo simulation.

Ionization cross section data of nitrogen, methane, and propane for light ions and electrons and their suitability for use in track structure simulations

“…Simulations with the PTRA code have shown to reproduce well the probability distribution of ionization cluster sizes produced by 4.6-MeV alpha particles in volumes of nitrogen, measured with another nanodosimeter known as the Jet Counter [4]. Furthermore, experimental cluster size distributions in propane obtained with an ion-counting nanodosimeter, similar to that of PTB/WIS, for 4.3-MeV alpha particles and protons of energies above 7 MeV also showed good agreement with calculated data [7].…”

“…Durić et al [21] C H 4 , C 3 H 8 12-240 Grill et al [24] C H 4 15-950 Nishimura et al [23] C H 4 , C 3 H 8 15-3000 Schram et al [22] C H 4 , C 3 H 8 600-12 000 Theoretical De Souza et al [25] C H 4 2-500 Vinodkumar et al [26] C H 4 15-2000 Models BEB model [19] (see Appendix A) Chouki model [27] (see Appendix B) Table I). Error bars are only shown for selected data points to improve readability.…”

“…The Physikalisch-Technische Bundesanstalt (PTB) track structure code "PTRA" is dedicated for applications in nanodosimetry [4,8]. This code also models the experimental setup of the ion-counting nanodosimeter which has been developed by the PTB and Weizmann Institute of Science (WIS) and has been comprehensively described in [7].…”

“…The total and differential cross section data for the processes described above, previously implemented in PTRA, are summarized in Refs. [4,8,13].…”

“…Micro-and nanodosimetric approaches have been under development for several years [1][2][3][4][5][6] as a means to characterize the track structure of ionizing radiation. This characterization is particularly important for an estimation of initial radiationinduced biological effects on the microscopic scale.…”